1. Field of the Invention
The present invention is generally related to increasing the corrosion resistance of austenitic alloys such as nickel-based alloy materials, and more particularly to the formation of a chromium-rich, protective oxide layer on the surface of nickel-based alloy tubing.
2. Description of the Related Art
Nickel-based alloys containing chromium, such as Alloy 600 (UNS designation N06600) and Alloy 690 (UNS designation N06690), are commonly used in nuclear reactor systems, for example as tubing in nuclear steam generators. Release of nickel from the tubing during operation contributes to radiation fields in the primary circuits of water-cooled nuclear reactors. This is undesirable, since it increases the exposure of service personnel to radiation during maintenance.
The formation of an oxide layer on materials used in a nuclear reactor environment is known to inhibit corrosion during operation, thereby reducing radiation levels. Chromium-rich oxide surface layers are especially desirable, since they form self-healing, protective surface layers on nickel-based alloys. Iron oxide and nickel oxide layers on nickel-based alloys are not self-healing, and are therefore less desirable than chromium oxide layers. In addition, a chromium-rich oxide is a more effective barrier to the transport of nickel from the base metal. Thus the reduction of nickel release through controlled oxidation, or passivation, to produce a chromium-rich surface is a desirable goal.
Oxide layers can be formed on metal surfaces by exposure to aqueous environments at low to moderate temperatures, or by exposure to gaseous environments at moderate to high temperatures. Because of a focus on the treatment of tubing in completed and installed steam generators, efforts within the industry have been directed primarily toward aqueous oxidation processes or moderate temperature steam oxidation. Processes are known to build up a protective oxide layer on an Alloy 690 tube surface by exposing the surface to an aqueous solution containing lithium and hydrogen at 300xc2x0 C. for 150 to 300 hours, or by exposure to wet air at 300xc2x0 C. for 150 to 300 hours. In another known process, Alloy 690 surfaces are exposed to a gaseous Arxe2x80x94O2xe2x80x94H2 mixture at intermediate temperatures of 573 to 873xc2x0 K. (300-600xc2x0 C.) for times between 15 and 480 minutes in a microwave post-discharge to produce a chromium-rich, protective oxide layer.
The above approaches suffer from long processing times and may impose risks to completed vessels during processing. A further problem is the relatively thin oxide layer [typically 10-50 nm and usually  less than 100 nm] that is formed.
Austenitic alloys containing appreciable amounts of chromium are often annealed under conditions specifically selected to retain a bright surface condition, with little or no oxidation or discoloration. The annealing process conditions are normally chosen to minimize oxide formation, rather than to deliberately produce an oxide of controlled thickness. A common way of achieving this is to use hydrogen gas with a very low moisture content, as measured by a low dew point of xe2x88x9240xc2x0 C. or lower, during the annealing process.
From the preceding discussion it is apparent, that a rapid method for producing a protective layer on nickel-based alloys would be welcomed by industry.
The present invention employs a controlled mixture of water in otherwise pure non-oxidizing gas to produce a protective, chromium-rich layer on a nickel-based alloy workpiece containing chromium, such as Alloy 600 and Alloy 690 nuclear steam generator tubing. The chromium-rich layer is produced from chromium already present in the workpiece. No external sources of chromium are required eliminating the need to buy, handle and dispose of unused amounts of this potentially hazardous material. The relatively thick chromium oxide layer provides a long term barrier to the release of nickel. The process conditions of the invention are compatible with high temperature annealing manufacturing steps. The invention can therefore be practiced simultaneously or in conjunction with high temperature annealing operations, for example during the manufacture of nuclear steam generator tubing. The invention thus provides a rapid and low cost method of passivating a nickel-based alloy workpiece containing chromium and preventing release of nickel into nuclear reactor primary coolant, while maintaining short construction schedules. Performing the passivation during tube manufacture also avoids the risks and penalties of passivating tubing in the finished vessel.
Accordingly one aspect of the present invention is drawn to a method of forming a chromium-rich layer on a surface of a nickel-based alloy workpiece that contains chromium. The chromium contained in the workpiece is oxidized by heating the workpiece to a temperature sufficient to oxidize the chromium, and exposing the workpiece to a gaseous mixture of water vapor and one or more non-oxidizing gases.
Another aspect of the invention is drawn to a method of forming a chromium-rich layer, including chromium oxide, on a surface of a nickel-based alloy workpiece that contains chromium, by heating the workpiece to a temperature of about 1100xc2x0 C., and exposing the surface of the workpiece to a flowing gaseous mixture of hydrogen and water having a water content in the range of about 0.5% to 10% for at least about 3 to 5 minutes.
Yet another aspect of the invention is drawn to a method of forming a chromium-rich layer consisting essentially of chromium oxide, on a surface of a nickel-based alloy workpiece that contains chromium, by heating the workpiece to a temperature of about 1100xc2x0 C., and exposing the surface of the workpiece to a flowing gaseous mixture of hydrogen and water having a water content in the range of about 0.5% to 10% for at least about 3 to 5 minutes.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by it use, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.